Our method constructs composite images of outer surfaces of spinning or looping objects. We use standard, relatively inexpensive, commercially available video processing equipment. We build upon and modify the basic method of U.S. Pat. No. 4,970,597 to Shepard entitled "Method to Image High Speed Events Using A Standard Video Camera," which constructs composite images of high speed events.
In U.S. Pat. No. 4,970,597, a series of individual video camera line signals are selected from video frames imaging recurrences of a repeating event. U.S. Pat. No. 4,970,597 uses an aperture signal 36 whose signal points 38 occur a predetermined time before a video camera line signal begins. A flagging signal 32 occurs a constant time "c" before each recurrence of the event. In essence, the temporal juxtaposition of flagging signal 32 and a signal point 38 indicate how close in time the recurrence is to the onset of a camera video line signal. If the recurrence is close enough to the onset of the line signal, then the line signal is used in constructing a composite image of the event.
Instead of a flagging signal, we use rotational position signals generated by a shaft encoder. Each rotational position signal corresponds to one of a multiplicity of known angular positions of a spinning object such as a tire. Each rotational signal can be regarded as an indicator of a recurrence of an event, which is the spinning object's arrival at one of its known angular positions. If the event occurs within a selected time of the beginning of a given video line signal from a camera, then the given line signal is a candidate for use in construction of a composite image of the spinning object. The given line must meet further criteria of our method before it is used for the composite image.